In order to realize multi-scale PIIFD, local information of feature points needs to be extracted in the scale space of the images. The method of building image's Gaussian pyramids based on the scale-space theory [1]} is adopted. The schematic diagram of establishing Gaussian pyramid of image in the proposed algorithm is shown in Fig. 4. The original image is sampled down step by step to obtain a series of images with different scales, that is, the first layer in each octave. Then a series of Gaussian blurs are performed on each octave:
\({\bf {L}} = {\bf {G}} * {\bf {I}}\) 
\({\bf {G}} = \frac{1}{{\sqrt{2\pi {\sigma ^2}} }}{e^{\frac{{ - ({x^2} + {y^2})}}{{2{\sigma ^2}}}}}\) 
